Katherine Russell, who continues to face questions from the US authorities, added Joshua Dratel to her legal team, her lawyer Amato DeLuca said. Dratel's "unique, specialised experience" will help ensure that Russell "can assist in the ongoing investigation in the most constructive way possible", said DeLuca.

Dratel has represented a number of terrorism suspects in US federal courts and military commissions, including Guantánamo Bay detainee David Hicks, who attended an al-Qaida-linked training camp in Afghanistan.

DeLuca, who specialises in civil cases such as personal injury law, said Russell will continue to meet investigators as "part of a series of meetings over many hours where she has answered questions".

An FBI spokeswoman would not comment when asked on Wednesday whether Russell is co-operating with the authorities. DeLuca has said his client had no reason to suspect her husband and his brother in the bombing.

Tsarnaev and his brother, Dzhokhar, are accused of planting two bombs near the finish line of last month's Boston Marathon, killing three people and injuring about 260. The brothers were ethnic Chechens living in Massachusetts.

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